A lithium-ion secondary battery (hereinafter sometimes referred to as a “lithium battery”) has characteristics that it has a higher energy density than other secondary batteries and is operable at a high voltage. Therefore, it is used in information devices such as cellular phones, as being a secondary battery which can be easily reduced in size and weight, and in recent years there have also been increasing demands of the lithium-ion secondary battery to be used as a power source for large-scale apparatuses such as electric vehicles and hybrid vehicles.
The lithium-ion secondary battery comprises: a cathode; an anode; and an electrolyte layer arranged therebetween. In general, an electrolyte to be used for the electrolyte layer is a nonaqueous liquid material or a solid material, for example. When the liquid electrolyte (hereinafter referred to as an “electrolytic solution”) is used, it easily permeates into the cathode and the anode. Therefore, an interface can be easily formed between an active material contained in the cathode or the anode and the electrolytic solution, and the battery performance can be easily improved. However, since the electrolytic solution, which is widely used, is flammable, it is necessary to mount a system to ensure safety. On the other hand, using the electrolyte in solid form (hereinafter referred to as a “solid electrolyte”), which is nonflammable, makes it possible to simplify the above system. Therefore, development of a lithium-ion secondary battery provided with a layer containing the solid electrolyte has been promoted (hereinafter, the layer will be referred to as a “solid electrolyte layer”, and the battery will be referred to as a “solid battery”).
As a technique on such a lithium-ion secondary battery, Patent Document 1 for example discloses a manufacturing method of a lithium battery comprising the steps of: preparing an active material slurry by dispersing an active material in a solvent containing a lithium-ion conductive binder; preparing a solid electrolyte slurry by dispersing a sulfide-based solid electrolyte in a solvent containing a lithium-ion conductive binder; and forming an active material sheet and a solid electrolyte sheet by dripping the active material slurry and the solid electrolyte slurry, respectively, on a base material with side guards, adjusting the thickness of the slurry with a blade, and also heat-drying and peeling off the slurry. In addition, Patent Document 2 discloses a production method of a solid electrolyte layer comprising the steps of: preparing a slurry for forming a solid electrolyte layer, by mixing a sulfide solid electrolyte material with a dispersant made of at least one selected from tertiary amine, ether, thiol, ester having a functional group of a carbon number of three or more bonded to the carbon atom of the ester group and a functional group of a carbon number of four or more bonded to the oxygen atom of the ester group, and ester having a benzene ring bonded to the carbon atom of the ester group; making a coating film for forming a solid electrolyte layer by applying the slurry for forming a solid electrolyte layer onto a base material; and forming a solid electrolyte layer by drying the coating layer for forming a solid electrolyte layer. Furthermore, Non-Patent Document 1 mentions that after immersing a sulfide electrolyte in a solvent and examining the lithium-ion conductivity thereof and the reactivity thereof with the solvent, it was found that a solvent to be used in a wet process was limited to a non-polar solvent such as toluene and heptane.